One relatively common type of intrusion detector system employs a pulsed beam of infrared radiation passing through an area to be protected. When the beam is broken by an intruder, a suitable alarm is actuated. The infrared beam is formed by a transmitter and received by a receiver. In one well-known type of installation, both the transmitter and the receiver are designed to be inconspicuous and resemble, upon casual inspection, a conventional duplex electrical outlet. Each is recessed into a wall and covered by a conventional duplex outlet plate. One of the openings in the outlet plate defines a window for transmitting or receiving the infrared beam. The openings are backed by a filter plate which is substantially opaque to visible light but is transparent to infrared.
The mechanical construction of the transmitter and receiver units is substantially identical, although one includes a radiation source such as a light emitting diode while the other includes a radiation detector. Each unit has a mirror placed immediately behind the window at an angle of approximately 45.degree. to the horizontal and a lens which is fixedly mounted directly below the mirror. The lens is a collimator and at its focal point is either a radiation source (in the case of a transmitter) or a radiation detector (in the case of a receiver). The mirror is designed to be pivoted about a vertical axis which is also the optical axis of the lens. In this matter it can be caused to "look" in a direction up to 45.degree. to the surface of the wall in which the unit is mounted. By means of such a construction, it is possible for the invisible radiation beam to pass between two units which are not opposed to each other.
A problem with units constructed in accordance with the prior art as described above is that, as the mirror is rotated to one side or the other, its field of view is increasingly obstructed by the opening in the outlet plate. As a result of such attenuation, it has been necessary to increase the sensitivity of the electronic circuitry in compensation. This, of course, increases the cost of the circuitry. It was also desired to provide a terminal block in the front of the unit. This required that the focal point of the lens be moved farther to the rear of the unit. However, it was not desirable to move the mirror to the rear as well, since this would only compound the problem of beam attenuation.
Accordingly, it is a primary object of the present invention to provide an adjustable mirror mounting which produces less attenuation of the beam by the radiation window when positioned for oblique viewing. Other objects are to provide a unit which requires less sensitive electronic circuitry and wherein the focal point is movable to the rear of the unit without increasing beam attenuation. The manner in which these objects are achieved will be apparent from the following description and appended claims.